1. Field of the Invention
The present invention relates to an electronic transmission control system for an automotive vehicle employing a belt-type continuously variable automatic transmission (often abbreviated to "CVT") which uses a drive belt running in a pair of variable-width pulleys to provide varying gear ratios (transmission ratios or pulley ratios), and specifically to techniques for directly efficiently controlling working pressures of adjustable flanges (or axially-slidable flanged pistons) of both primary and secondary pulleys of the CVT by the use of two motor-driven oil pumps.
2. Description of the Prior Art
Japanese Patent Provisional Publication No. 7-259941 has disclosed a transmission control system for a belt-type continuously variable automatic transmission (CVT). Generally, the V groove of a primary pulley of the belt-type CVT is constructed by a stationary flange and an adjustable flange axially slidable on linear ball bearing splines for varying the width of the V groove of the primary pulley by hydraulic pressure (often called a "primary pulley pressure" or a "speed-change control pressure"), whereas the V groove of a secondary pulley is constructed by a stationary flange and an adjustable flange axially slidable on linear ball bearing splines for varying the width of the V groove of the secondary pulley by at least hydraulic pressure (often called a "secondary pulley pressure" or a "regulated line pressure"). The CVT has a hydraulic modulator or a hydraulic control unit for controlling the working pressure of the adjustable flange (the flanged piston) of each of the primary and secondary-driving and driven-pulleys. The hydraulic control unit includes a line pressure control valve which regulates the hydraulic pressure of working fluid discharged from a sole oil pump and then produces the regulated line pressure (the secondary pulley pressure P.sub.sec) applied to a secondary pulley actuation cylinder, and a speed-change control valve which further regulates the line pressure and then generates the speed-change control pressure (the primary pulley pressure P.sub.pri) applied to a primary pulley actuation cylinder. The sole oil pump is commonly driven by an engine. The line pressure is regulated so that there is no slippage of a drive belt (usually a single segmented steel belt) running in the primary and secondary pulleys.